Tapwater Exposures, Effects Potential, and Residential Risk Management in Northern Plains Nations

In the United States (US), private-supply tapwater (TW) is rarely monitored. This data gap undermines individual/community risk-management decision-making, leading to an increased probability of unrecognized contaminant exposures in rural and remote locations that rely on private wells. We assessed point-of-use (POU) TW in three northern plains Tribal Nations, where ongoing TW arsenic (As) interventions include expansion of small community water systems and POU adsorptive-media treatment for Strong Heart Water Study participants. Samples from 34 private-well and 22 public-supply sites were analyzed for 476 organics, 34 inorganics, and 3 in vitro bioactivities. 63 organics and 30 inorganics were detected. Arsenic, uranium (U), and lead (Pb) were detected in 54%, 43%, and 20% of samples, respectively. Concentrations equivalent to public-supply maximum contaminant level(s) (MCL) were exceeded only in untreated private-well samples (As 47%, U 3%). Precautionary health-based screening levels were exceeded frequently, due to inorganics in private supplies and chlorine-based disinfection byproducts in public supplies. The results indicate that simultaneous exposures to co-occurring TW contaminants are common, warranting consideration of expanded source, point-of-entry, or POU treatment(s). This study illustrates the importance of increased monitoring of private-well TW, employing a broad, environmentally informative analytical scope, to reduce the risks of unrecognized contaminant exposures.


INTRODUCTION
Water is life (Mníwicȟońi in Lakota). The quality and sustainability of drinking water are growing challenges in the United States (US) and world wide due to, among other reasons, increasing water demands and drinking-water source contamination. 1−3 US public, private, and bottled drinking-water supplies share many anthropogenic-contaminant (i.e., humangenerated/-driven) concerns, because most of the 350,000 chemicals estimated to be in commercial use globally 4 and, by extension, potentially in drinking-water source waters 5,6 are not currently regulated or systematically monitored in public-supply tapwater (TW) 7,8 or in bottled drinking water. 9 Note, the U.S. Environmental Protection Agency (EPA) does not regulate private wells nor does it provide recommended criteria or standards for individual wells. 10 In contrast, many water-borne pathogens (e.g., Cryptosporidium) and naturally occurring contaminants (e.g., arsenic, As) are actively regulated in US public and bottled-water supplies. 7−9 The potential for unrecognized contaminant exposures and adverse health effects is notably elevated for private and small community drinking-water supplies in rural and remote areas, 11,12 due to differences in regulation, technical expertise, natural/economic resources, and associated risk-management options. 13−15 Although some geogenic groundwater contaminants, such as As, exhibit broadly predictive geospatial patterns, 16,17 individual point-of-use (POU) TW concentrations reflect multiple factors including well-screen/open-hole depth, geologic stratigraphy, and spatial/temporal variability in water levels and redox conditions and, consequently, can differ substantially and unpredictably from well to well. 18,19 Further, As and several other contaminants are imperceptible (tasteless and odorless) without TW testing. 11,12,20 Prior studies have documented a range of contaminant concerns in unregulated drinking water. 12,20,21 Due to high analytical costs, common-place conflation of organoleptic quality with safety, and other socioeconomic factors, privatewell water-quality data remain scarce and, where available, are typically limited to a few targeted contaminants. 20−23 Similarly, small rural water systems are often concerns for various socioeconomic reasons, including financial limitations of smaller and lower-income populations. 24−27 The elevated probability of unrecognized exposures has prompted calls for universal privatewell As testing, 23,28 but, acknowledging the increasingly humanimpacted water cycle, 29,30 broader characterization of privatewell and rural-water-system contaminant exposures is needed. 31 The Strong Heart Study (SHS) is an ongoing populationbased prospective-cohort study of cardiovascular disease and associated risk factors among American Indian (AI) adults in participating Tribal Nations in Arizona, North Dakota, Oklahoma, and South Dakota. 32 Groundwater As varies regionally across the US, with elevated concentrations predicted and observed 16,17,33,34 throughout SHS areas. SHS research has associated low to moderate drinking-water As exposures with lung, prostate, and pancreatic cancer, 35 cardiovascular disease, 36 kidney disease, 37 lung disease, 38 and type-2 diabetes outcomes. 39,40 Efforts to manage As-related health risks in SHS-area TW include the development/expansion of rural water systems and the Strong Heart Water Study (SHWS), a participatory randomized controlled intervention to reduce As exposures in remote residences in North and South Dakota using undersink adsorptive-media POU treatment. 41, 42 The U.S. Geological Survey (USGS) collaborates with EPA, the National Institute of Environmental Health Science (NIEHS), Food and Drug Administration (FDA), Tribal Nations, universities, utilities, communities, and others to inform drinking-water exposure and water-supply data gaps by assessing TW inorganic/organic contaminant mixtures and associated distal (e.g., ambient source water) and proximal (e.g., premise plumbing and POU treatment) drivers in a range of socioeconomic and source-water vulnerability settings across the US. 29,30,43,44 As part of that ongoing effort, we assessed exposures to a broad suite of potential inorganic and organic TW contaminants in 56 homes in three areas of North Dakota and South Dakota in 2019 to provide insight into cumulative contaminant risk to human health 45−47 of private-well and small community public-supply TW in this region, to assess the utility of broader TW contaminant assessments for identifying additional risk management efficiencies in areas undergoing drinking-water interventions for As, and to expand the national perspective on inorganic and organic contaminant exposures at the TW point of use by maintaining the same general sampling protocol and analytical toolbox employed in previous studies. 29,30,43,44 For this study, TW exposure was operationally represented as concentrations of 476 organics, 34 inorganics, and 3 bioactivity indicators in residential and community POU TW samples. Potential human-health risks of individual and aggregate TW exposures were explored based on cumulative detections/ concentrations of designed-bioactive chemicals (e.g., pesticides and pharmaceuticals), 5 as well as the cumulative exposureactivity ratio(s) (Σ EAR ) 48 and hazard indices (HI) 49,50 of cumulative benchmark-based toxicity quotient(s) (TQ) (Σ TQ ). 51 In line with previous results by this research group and others, simultaneous TW exposures to multiple inorganic and organic constituents of potential human-health interest were hypothesized to occur in both private-and public-supply samples. 12  private-supply (unshaded) TW samples within three study areas. Solid red lines indicate public-supply enforceable maximum contaminant level(s) (MCL) or technology treatment action level (Pb). MCL goals (MCLG) for As, U, and Pb are zero. The solid purple line indicates the Mn lifetime drinking water health advisory. Boxes, centerlines, and whiskers indicate interquartile range, median, and 5th and 95th percentiles, respectively. Numbers above each boxplot pair indicate the permuted probability that the centroids and dispersions are the same (PERMANOVA; 9999 permutations). "nd" indicates not detected. private; 10 private/10 public; and 8 private/12 public TW locations from communities A, B, and C, respectively (Figures 1 and S1). All community A locations were SHWS participants, for which the inclusion criterion was greater than maximum contaminant level(s) (MCL) baseline TW As concentrations. In this study, all private-supply and 15 public-supply TW samples were groundwater sourced. In community B, three and seven public-supply sample locations were connected to groundwatersourced and surface-water-sourced (Missouri River) water systems, respectively. Community C public-supply samples were groundwater sourced, with 11 samples from the primary, multiwell water system and one sample from a small (65 people) system. In general, TW samples were collected from untreated kitchen faucets, except for four (three private, one public) community B samples collected from POU-treatment taps (denoted POU-AF in Supporting Information Tables) at the participants' request. One other TW-sample location (com-munity B) had a point-of-entry water-softener system (POE-S in Supporting Information Tables).
Taps (cold water) were sampled once each from September to November 2019. Samples were collected at the participant's convenience throughout the day, without precleaning, screen removal, or Lead and Copper Rule stagnant-sample protocols. 52, 53 Complete sampling details are provided elsewhere. 54−56 2.2. Methods and Quality Assurance. Briefly, TW samples were analyzed by USGS using seven organic (6 classes; 476 total/468 unique analytes), five inorganic (34 ions/trace elements), and two field (3 parameters) methods (Table S2) and by USEPA using three in vitro bioassay (ER, AR, and GR) methods, as discussed 29,30,43,54 and described in detail previously. 57−74 Organic analytes included cyanotoxin, disinfection byproduct(s) (DBP), pesticide, per/polyfluoroalkyl substance(s) (PFAS), volatile organic compound(s) (VOC), and pharmaceutical classes; additional method details are in the Supporting Information. All results are in Tables S3−S4 and S8 and in Romanok et al. 55,56 Quantitative (≥limit of quantitation, ≥LOQ) and semiquantitative (between LOQ and long-term method detection limit, MDL 75,76 ) results were treated as detections. 75,77,78 Quality-assurance/quality-control included analyses of six field blanks, as well as laboratory blanks, spikes, and stable-isotope surrogates. Only bromide was detected in inorganic blanks at concentrations in the range observed in TW samples; results were censored at the maximum blank concentration (0.14 mg L −1 ), as footnoted (Tables S3 and  S5). Among detected organics, only ethyl acetate (0.14 μg L −1 ) was detected in a blank (1) in the concentration range observed in TW samples; results were censored at two times the maximum blank concentration (0.28 μg L −1 ), as footnoted (Tables S4 and  S6). The median surrogate recovery (Table S7) was 99.9% (interquartile range: 89−110%) 2.3. Statistics and Risk Assessments. Differences between TW-sample groups were assessed by nonparametric one-way PERMANOVA (n = 9999 permutations) on Euclidean distance (Paleontological Statistics, PAST, vers. 4.03). 79 Relations between detected TW contaminants were assessed by Spearman rank-order correlation (ρ) and permuted (n = 9999) probabilities (PAST, vers. 4.03). 79 Screening-level assessments 49,50 of potential cumulative effects were based on the cumulative exposure-activity ratio(s) (Σ EAR ) 48 and HI 49,50,80 of cumulative benchmark-based TQ (Σ TQ ), 51 as described. 29 ToxEval version 1.2.0 81 of R 82 was used to sum (noninteractive concentration addition model 83−85 ) individual ToxCast-based 86,87 exposure activity ratios (EAR) or benchmark-based TQ, respectively. For the latter, the most protective human-health benchmark (i.e., lowest benchmark concentration) among MCL goal(s) (MCLG), 7,88 WHO guideline values (GV) and provisional GV, 89 USGS Health-Based Screening Level (HBSL), 90 and state drinking-water MCL or health advisories (DWHA) was used. MCLG values of zero (i.e., no identified safe-exposure level for sensitive subpopulations, including infants, children, the elderly, and those with compromised immune systems and chronic diseases 7,91 ) were set equal to the method reporting limit or 1 μg L −1 for lead (Pb). 92 Cumulative EAR (Σ EAR ) and TQ (Σ TQ ) results, ToxCast exclusions, and health-based benchmarks are summarized in Tables S9−S13 (additional details in Supporting Information).

RESULTS AND DISCUSSION
Regulated and unregulated chemicals (inorganic, organic) were detected in TW samples in all three study areas (Tables 1, S3, and S4; Figures 1−4), with two or more detections of humanhealth interest commonly observed per sample. 63 (14%) organic and 30 (91%) inorganic analytes were detected. In this discussion, the enforceable EPA MCL or, for Pb, action level (AL) for technology treatment is provided from a regulatory perspective, but the organ/organism-level human-health effects of individual contaminant exposures are contextualized based on MCLG and human-health advisories, for three reasons. First, EPA MCL and AL are enforceable in public supplies 7,8 but not in private supplies. 10 Second, MCL and AL are set as close to MCLG values as feasible but are often higher to reflect technical and financial constraints of drinking-water monitoring and treatment. 91 Last, MCLG and health advisory values generally include a margin of exposure to provide a safety threshold, in the case of MCLG defined as the concentration below which there is no known risk to the health of presumptive "most vulnerable" (e.g., infants, children, pregnant women, elderly, and immunecompromised) subpopulations. 91 3.1. TW Arsenic, Uranium, and Lead. Consistent with previous reports, 33,34,41,93 TW exposures to As, U, and Pb, which have no known safe level of drinking-water exposure for vulnerable sub-populations (MCLG zero), were widely observed (54, 43, and 20% of samples, respectively) in the three study areas in private-and public-supply locations ( Figure  2).
In line with earlier SHWS findings, 41 the redox-reactive geogenic contaminant As was consistently detected (MCLG exceedance) in communities A and C (94 and 75% of samples, respectively), but not in community B. Consistent with the SHWS inclusion criterion, As was detected in all but one community A sample, near (9 μg L −1 in three samples) or above (11 samples) MCL-equivalent concentrations in 88% (14/16) of samples. In community C, As was detected in 50% (4/8) and 92% (11/12) of private-and public-supply samples, respectively, but MCL-equivalent exceedances occurred only in three privatewell locations. Overall, the results support ongoing expansions/ connections to public supplies as an effective approach to limit TW As-exposures to <MCL concentrations in these communities and POU/POE treatment 41,42 in private-supply locations where a public-supply connection is not feasible. The results emphasize the potential for unrecognized exposures to contaminants, including contaminants of human-health concern, in unregulated/unmonitored private supplies and support private-well As monitoring. 23,28, 33 The results also support the potential use of POU/POE treatment at public-supply locations with detectable As, as an additional As removal step. While Asassociated Safe Drinking Water Information System (SDWIS) violations, 94 As concentrations, and urinary As levels 95,96 associated with public supplies have decreased substantially since the MCL was lowered, no safe level of TW As exposure is recognized for vulnerable subpopulations. 97 Drinking water As exposure is associated with various cancers, 98,99 organ-system toxicity, 98 cardiovascular diseases, 100,101 diabetes, 100,101 adverse pregnancy outcomes, 102 and mortality. 102,103 Adverse health associations with <MCL As exposures 35,98,100,104 have prompted a 5 μg L −1 MCL in some US states (e.g., New Jersey and New Hampshire 105,106 ). Every private-well sample with detectable As and 92% of community C public-supply samples exceeded 5 μg L −1 .
Also, consistent with previous findings, 34,93 the redox-reactive geogenic radionuclide U was detected (MCLG exceedance) in all three communities (A: 94%, B: 25%, and C: 20%). Drinkingwater U is associated with nephrotoxicity 107,108 and osteotoxicity 109 in humans, inhibition of DNA-repair mechanisms in human embryonic kidney 293 (HEK293) cells, 110 and estrogenreceptor effects in mice. 111 The MCL-equivalent concentration was exceeded only in one private-well location. Community C U exposures differed (PERMANOVA, p = 0.013) between private-(4 detections) and public-supply (no detections) locations. No difference was apparent in community B (p = 0.93), with 2−3 detections (≤6 μg L −1 ) each in private-well and public-supply samples. Other widely documented, drinking-water radionuclides (e.g., radium and radon) 6,21 not assessed herein should be included in future assessments.
As and U co-occurred in 16 (42%) of the 38 samples in which either was detected and were moderately positively correlated (Spearman ρ = 0.48, permutation [n = 9999] p = 0.0002). Because groundwater As and U are favored under reducing and oxidizing conditions, respectively, frequent co-occurrence in SHWS well water suggests redox heterogeneity is common 34 or redox-independent mechanisms (e.g., pH/alkalinity-driven solubility) are important. 93 Elevated iron (Fe) and manganese (Mn) concentrations in oxic TW herein support the former. A moderate correlation (ρ ≥ 0.42; p < 0.0001) between U and nitrate (NO 3 ) was reported 112 for the High Plains aquifer, the northern extension of which underlies community A; a weaker correlation (ρ ≥ 0.27; p < 0.042) was observed herein.
Pb was detected sporadically (20% of samples) in private-and public-supply locations in all communities but did not exceed the AL-equivalent concentration. Elevated drinking-water Pbexposures primarily are associated with neurocognitive impairment in infants and children. 92,113 The American Academy of Pediatrics 92 recommends that drinking-water Pb not exceed 1 μg L −1 , a common MDL for US public-supply compliance monitoring. 52 Drinking-water Pb is attributed primarily to premise-plumbing and distribution-infrastructure materials 113 that predate the 1986 SDWA Amendments. 114 In this study, no difference in Pb detections (p ≥ 0.11) between private-and public-supplies and no systematic Pb detections within a given public-supply system support premise plumbing as the probable source. Importantly, the current results likely underestimate TW Pb occurrence and concentrations in the study area, because flushing effectively decreases plumbing-derived contaminant  134 1 μg/L 0  N) concentrations were generally low (median: 0.4 mg L −1 ) in the study area, with elevated (>2 mg L −1 ) concentrations, including one MCL-equivalent exceedance (Table 1), observed infrequently (4 samples) and only in private-well locations. No cocontaminants (e.g., human-use pharmaceuticals) indicative of human-waste sources (e.g., septic systems) were detected in elevated NO 3 −N samples, indicating other sources, such as inorganic/organic crop fertilizers or animal wastes, as possible contributors to elevated TW NO 3 −N concentrations; the lack of detectable pesticides in the four TW samples with >2 mg L −1 NO 3 −N points toward the latter. The NO 3 −N MCLG was established to protect against bottle-fed infant (<6 months) methemoglobinemia. 7 Emerging evidence associates <MCL NO 3 −N concentrations with other adverse health outcomes, 116 including cancer, 117 thyroid disease, 118 and neural tube defects. 119 3.3. TW Manganese. No Mn MCL 97 (or WHO GV 89 ) currently exists, but USEPA maintains a 300 μg L −1 lifetime drinking-water health advisory (DWHA; assumes 100% exposure from drinking water). 97 Approximately 32% (18/56) of the samples in this study (75% in community C) exceeded or equaled the USEPA DWHA, raising concerns about prolonged exposures. More concerning, a Mn concentration greater than the 1 day acute exposure level of 1 mg L −1 for small (≤10 kg) children 97 co-occurred in a private-well TW sample with a greater than MCL-equivalent U concentration, again emphasizing the elevated risk of unrecognized exposures in private-well TW. Community B Mn concentrations differed by an order of magnitude (p = 0.0002) between surface-water-sourced publicsupply (median: 2 μg L −1 ) and groundwater-sourced (private-/ public-supply median: 23 μg L −1 ) samples. Median Mn concentrations were more than an order of magnitude higher in community C samples (all groundwater-sourced), with no difference (p = 0.21) between private-and public-supply locations (medians 575 and 554 μg L −1 , respectively). However, the highest Mn concentration in this study (2880 μg L −1 ) was observed in a private-well TW sample in community C, which also had the study's highest dissolved Fe (4640 μg L −1 ) and only >MCL-equivalent U concentration (34 μg L −1 ). As noted above, frequent co-occurrence in groundwater-sourced TW samples of redox-reactive inorganics, including those (e.g., As and U) mobilized under different redox conditions, is consistent with widespread well-water redox heterogeneity. 34 Across the US, approximately 6.9% of drinking-water-aquifer samples (n = 3662) exceeded 300 μg L −1 Mn, an exceedance rate comparable to those for MCL-equivalent concentrations of NO 3 −N (4.1%) and As (6.7%) in the same wells. 6 Groundwater Mn concentrations in excess of 300 μg L −1 were associated with proximity to surface waters and organic-rich soils, 120 consistent with the elevated Mn concentrations observed in community C, near a large regional lake. Growing concerns for cognitive, neurodevelopmental, and behavioral effects of long-term exposures in children have prompted calls to re-evaluate the regulation/monitoring of drinking-water Mn. 121,122 To protect against neurological effects in bottle-fed infants, WHO 123 recently released a provisional Mn GV of 80 μg L −1 , a value

TW Selenium.
Elevated drinking-water Se has been proposed as a risk factor for adverse health outcomes, 124 including amyotrophic lateral sclerosis (ALS), Parkinson's disease, 125 neurotoxicity, 126 and skin cancer. 127 Elevated serum Se concentrations have been associated with diabetes and elevated fasting glucose. 128,129 While the USEPA Se MCLG is currently 50 μg L −1 , 97,130 growing concerns prompted a recent 20 μg L −1 European Commission parametric value for Se. 131 Based on a comparative cohort (exposed, unexposed) study of long-term exposure to drinking-water Se in the range of 7−10 μg L −1 in northern Italy, 132,133 a drinking-water limit of 1 μg L −1 was proposed to decrease the risk of adverse health effects, including neoplasms and endocrine and neurological diseases. 134 In the current study, Se was detected in excess of 1 μg L −1 in 26% (9/34) of private-well TW samples (8/14 community A), but not in any public-supply samples.
3.5. TW Fluoride. Detected F concentrations ( Figure S1, Table S3) were well below the USEPA MCL established to protect against bone fragility and skeletal fluorosis, 7,8 indicating little concern for this toxic effect from TW exposures within the study area. However, all F concentrations in the current study were below the US Public Health Service 135 optimum of 0.7 mg L −1 to prevent dental caries ( Figure S1), consistent with groundwater results 6,136 and dental-health concerns for children on private-wells 11 across the US. The American Academy of Pediatrics 137 and the Centers for Disease Control (CDC) 138 recommend F supplementation for children if their drinking water contains <0.6 mg L −1 F. 3.6. TW Organics. TW samples were screened for 4 cyanotoxins, 22 DBP, 218 pesticide, 34 PFAS, 78 VOC, and 112 pharmaceutical analytes. Among the 63 organic analytes detected at least once, 59% (38) were detected in 5% (≤3) or fewer samples, with 20 (31%) detected only once. At least one organic analyte was detected in 79% (44/56) of the TW sample locations, with more than one detected in 59% (33/56) of locations. Higher numbers and cumulative concentrations of detected organics were observed in public-supply TW samples than in private supply in communities B and C (p = 0.0001), attributable primarily to DBP. In general, few organics were detected in private-well samples (Figures 4 and 5), with 35% (12/34) having no organic detections; a median detection of 1 for communities A (range: 1−9), B (range: nd−4), and C (range: nd−6); and no systematic detections. However, the highest individual (isopropyl alcohol; Chemical Abstract Service [CAS] number: 67-63-0) and cumulative concentrations of detected organics (all VOC) were observed in a private-well TW sample from community A; isopropyl alcohol is a common household solvent and personal care product. 139 The VOC class dominated organic detections and concentrations in private-supply TW, on average (median) accounting for 100% of cumulative detections (IQR: 50−100%) and concentrations (IQR: 82−100%) in samples with detectable organics. The most frequent organic analyte detections in private-well TW (24%) were generally trace levels of 1,1difluoroethane (CAS: 75-37-6), most commonly used as an aerosol propellant; 139 similar frequency (27%) sporadic detections in public-supply samples are most readily reconciled with trace residential air contamination. The co-occurrence of multiple petroleum-hydrocarbon contaminants in two remote rural, private-well TW locations is consistent with groundwater contamination from farm-related refueling activities. VOC remained a frequently detected class in public-supply TW locations, with predominantly xylenes observed in community C and xylenes and the fuel-oxygenate, t-butyl alcohol, systematically detected in community B. However, consistent with previous findings, 29,30,43,44 DBP dominated public supply organics, representing on average (median) 58% of detections and 92% of cumulative concentrations. Cumulative DBP concentrations were more than an order of magnitude higher (p < 0.0001) in surface-water-sourced public-supply TW samples (community B only; median: 24.2 μg L −1 ) than in groundwater-sourced public-supply locations (median: 2.0 μg L −1 ) in communities B and C. Consistent with widespread agricultural use within Corn Belt drainage basins, 140,141 including the Missouri River basin, 142 pesticides (median: 6) were detected (concentrations ≤ 0.33 μg L −1 ) in every surfacewater-sourced public-supply sample but were rarely detected in any groundwater-sourced location in this study. Other notable, albeit less frequent, organic detections of growing human-health concerns included PFAS compounds (PFBA, one private supply, 83.7 ng L −1 ; PFBS, two private supplies, maximum 13.2 ng L −1 ) and cyanotoxins (cylindrospermopsin, one public supply, 90 ng L −1 ; saxitoxins, two private supplies, both 30 ng L −1 ), all below current lowest state drinking-water ALs or advisories for PFBA (7000 ng L −1 ), 143 PFBS (345 ng L −1 ), 144 cylindrospermopsin (700 ng L −1 ), 145 and saxitoxins (300 ng L −1 ). 145 Among the 63 detected organics, 16 have MCLG; among these 12 have individual MCL and 4 are addressed as a class by the trihalomethane (THM) MCL. No MCL or MCL-equivalent concentration was exceeded in public-or private-supply samples, respectively. However, seven detected analytes have an MCLG of zero. Among these MCLG exceedances, only bromodichloromethane, tribromomethane, and dichloromethane were detected in more than one sample and only in treated public supplies. The remaining MCLG (zero) exceedances were single detections each of 1,2-dichloropropane, benzene, trichloroethene (TCE), and vinyl chloride.
3.7. TW In Vitro ER, AR, and GR Bioactivity. While estrogenic activity was detected in five samples above the T47D-KBluc minimum detectable concentration [MDC; 0.0683 ng 17β-estradiol equivalents (E2Eq) L −1 ], and androgenic activity was detected in one sample above the CV1-chAR MDC (0.9 ng dihydrotestosterone equiv L −1 ), glucocorticoid activity was not detected above its bioassay MDC (Table S8). No detected activity (estrogen or androgen activity) exceeded respective drinking water effect-based trigger values (indicative of adverse health effects) previously developed for similar molecularendpoint bioassays. 146 3.8. TW Aggregated Screening Assessment: Σ EAR and Σ TQ . We screened for TW cumulative-exposure effects of potential human-health interest using two analogous bioactivityweighted approaches (Σ EAR , Σ TQ ) with distinct strengths and limitations. Both are based on detected TW constituents and constrained by the analytical scope (468 organics and 33 inorganics), which, while extensive, is an orders-of-magnitude underestimate of the contaminant exposures documented in drinking-water sources. Likewise, both approaches are limited to available weighting factors (ToxCast ACC and human-health benchmarks, respectively) and assume cumulative effects are reasonably approximated by concentration addition. 83, 85,147 The Σ EAR approach 29,48 leverages high-throughput exposureeffects data for the 10,000 + organics and approximately 1000 vertebrate-cell-line molecular endpoints 148,149 in the invi-ACS ES&T Water pubs.acs.org/estwater Article troDBv3.2 release 150 of the ToxCast database to estimate potential cumulative activity at sensitive and possibly more protective sublethal molecular endpoints but has limited to no coverage of inorganic contaminants and unknown transferability to organ/organism scales. 151 Importantly, the approach employed here aggregates contaminant bioactivity ratios across all endpoints without restriction to recognized modes of action as a precautionary screening for further investigation of potential effects but may not accurately reflect the apical effects that typically drive regulatory risk assessments. 48,151 In contrast, the Σ TQ HI approach provides insight into potential effects of simultaneous inorganic and organic exposures and is targeted at apical human-health effects, but it is notably constrained to available regulatory risk determinations.
Only about half (32) of the 63 organics detected in TW in this study and only 5 of 12 detected DBP had exact Chemical Abstract Services (CAS) number matches in the ToxCast invitroDBv3.2 database ( Figure S2; Table S10). Consistent with the presence of DBP in public supply and the generally infrequent and low-concentration detections of organics in private supplies, site-specific Σ EAR was higher (p = 0.0001) in public-supply TW (median: 0.0412) than in private supply (median: <0.00001) ( Figure 6; Table S10). However, the highest EAR (and Σ EAR ) in this study was observed in a privatewell TW sample from community A with a concentration of 1butanol more than 10 times that (ACC) shown to modulate molecular targets in vitro (i.e., solid red Σ EAR = 1 line). Individual EAR or Σ EAR near or above 0.1 in the seven surface-watersourced TW samples from community B indicated an elevated probability of effects, driven primarily by the DBP, chlorodibromomethane. Exceedance of Σ EAR = 0.001 (precautionary screening-level threshold of interest) in all public-supply and five private-supply samples indicated that further investigation of the cumulative biological activity from SHWS area TW exposures is warranted, even when considering only the organic contaminants detected in this study.
Every TW sample exceeded the Σ TQ = 0.1 HI screening threshold of concern and all, but four, private-well TW samples exceeded Σ TQ = 1 (Table S12). These Σ TQ results indicate high probabilities of aggregated risks in SHWS private-and publicsupply TW samples when considering exposures to both organic and inorganic chemicals (Figures 6 and S3; Table S13). Σ TQ was  Table S12 and detected in treated public-supply (shaded) and untreated private-supply (unshaded) TW samples. Solid and dashed red lines indicate benchmark equivalent concentrations and effects-screening-level threshold of concern (TQ = 0.1), respectively. Boxes, centerlines, and whiskers indicate interquartile range, median, and 5th and 95th percentiles, respectively, for both plots.

ACS ES&T Water
pubs.acs.org/estwater Article driven primarily by inorganics (As, Mn, Pb, and U) in privatewell TW samples and by organics (DBP) and inorganics (As, Mn, Pb, and U) in public-supply TW samples. Site-specific HI were higher (p = 0.0009) in public-supply (median: 30.6) than in private-supply (median: 20.3) TW (Figure 7), with only modest differences (p = 0.0301) in community C but approximately two orders-of-magnitude higher (p = 0.0001) median Σ TQ in community B public-supply (median: 89.6) versus private-supply (median: 0.82) samples. Common-place DBP-driven exceedance of HI screening-levels of concern in public-supply TW, in this study and previously, 29,30,43,44 reemphasize the public-health tradeoff of chlorine disinfection, 152,153 the importance of better understanding of the cumulative DBP health risks, 152,154 and the need for improved DBP-precursor (e.g., natural organic matter) removal prior to disinfection. 155,156 Likewise, frequent exceedances of the Σ TQ HI screening level of 0.1 in unregulated and generally unmonitored private-supply TW in this and previous studies 29,30,44 reiterate the inherent human-health challenge of unmonitored TW. 6,12,21−23 Simultaneous co-occurring inorganic and organic exposures and corresponding potentials for cumulative effects add weight to previous recommendations for systematic private-supply monitoring, 23 with an analytical scope that more realistically reflects the breadth of inorganic and organic environmental contamination. 5,157 3.9. Implications for Drinking-Water Treatment and TW Exposure Mitigation. The results indicate effective treatment of regulated contaminant exposures to below MCL levels in all public-supply TW samples in this study. Considering the multiple exceedances of health-only MCLG, 7,8 however, complete communication of public-supply TW monitoring results, including exposures below current MCL, is needed to support consumer POU-treatment decision-making. Likewise, the ongoing SHWS As intervention has documented an effective reduction of TW As exposures in private-well locations in the study area using POU adsorptive media. 41 In light of growing evidence for human-health effects of exposures to currently unregulated TW contaminants (e.g., unregulated DBP 154 and PFAS 158 ) or to regulated contaminants at <MCL concentrations (e.g., As 98 and NO 3 116 ), common co-occurrences of multiple analytes with human-health implications in both private-and public-supply samples, including co-occurring exceedances of MCLG and Σ TQ > 1, may reasonably raise consumer concerns 1,2 and corresponding interest in POE/POU treatment. 159 The median per sample number of healthbenchmark exceedances in this study was two (range: 0−5), illustrating the importance of identifying stand-alone POE/ POU treatment options for unregulated private-well TW, which are effective against multiple contaminants, 159,160 and highlighting the potential value of POU treatment of public-supply TW for additional contaminant removal, including DBP. 159 Several POE/POU treatment technologies are effective in reducing TW exposures to the contaminants identified in this study. 159 The protectiveness of POE/POU approaches depends on the selection of appropriate filtration technologies for exposures of concern, timely and effective maintenance, and monitoring to confirm acceptable performance. For locations in the study area where geogenic contaminants like As and U are the only exposures of apparent concern, single-stage configurations of multiple treatment technologies are appropriate, including solid-block activated carbon, ion exchange media, redox media, and reverse osmosis (RO). 159,161 However, broadly effective single-stage treatment technologies, such as RO, or multistage/multifiltration systems (sediment filter, redox media, activated carbon, ion exchange, RO, and UV disinfection) may be more appropriate for those locations Figure 7. Cumulative EAR (Σ EAR , left, organics only) and cumulative TQ (Σ TQ , right, inorganics and organics) for analytes detected in public-supply (shaded) and private-supply (unshaded) TW samples during 2019 in North Dakota and South Dakota within each study area. Solid and dashed red lines indicate in vitro effect or benchmark equivalent exposures (Σ EAR = 1, Σ TQ = 1) and effects-screening-level thresholds of concern (Σ EAR = 0.001, Σ TQ = 0.1), respectively. Boxes, centerlines, and whiskers indicate interquartile range, median, and 5th and 95th percentiles, respectively. Numbers above each boxplot pair indicate the permuted probability that the centroids and dispersions are the same (PERMANOVA; 9999 permutations). with mixtures of inorganic and organic contaminants or unknown contaminant-exposure profiles (i.e., unmonitored private wells). 159

CONCLUSIONS
Assessment and communication of TW contaminant exposures are essential to contaminant-risk-management and public-health decision-making at household and community scales. The perceived risks of and resultant resource commitments to drinking-water contaminant exposures are highly variable at both scales due to differences in availability of actionable drinking-water contaminant exposure/risk information, risk acceptability/tolerance thresholds, and overall exposure/risk portfolios (exposome). 153,162 In the US, SDWA-stipulated annual public-supply consumer confidence reports support community and household-level decision-making, but decisions concerning additional community-or household-level treatment of public-supply TW are constrained by limited information on unregulated contaminants and on <MCL (or <AL) concentrations of regulated contaminants. Limited financial resources, water-quality expertise, and contaminant exposure data at the household scale fundamentally constrain private-supply decision-making and risk-mitigation options. 23,29,44 Analytically extensive datasets like this study, which are intended to inform scientific and public-health understanding of the role of drinking water as a vector for human contaminant exposures and associated human-health outcomes, remain limited because broad assessment of regulated and unregulated contaminants are not generally conducted at the TW point of exposure in the US or worldwide.
These results indicate that simultaneous exposures to contaminants of human-health interest are common in both public-and private-supply TW locations across the study area. The bioactivity-weighted screening (Σ EAR and Σ TQ ) results (Tables S10 and S13) indicate that exposures to DBP and inorganics at <MCL concentrations are the primary drivers of human-health interest in public-supply TW in communities B and C. These public-supply results align with the previous findings 29,30,43,44 and support the need for better understanding of exposure-effect relations and cumulative health risks of regulated/unregulated DBP 152,154 and for improved sourcewater pretreatment technologies to address DBP precursors, such as surface-water natural organic matter. 156,163 Exposures to As and U in groundwater-sourced drinking-water supplies have been reported previously in the study area and the concentrations observed in public-supply TW herein were below corresponding NPDWR MCL. However, because there is no known safe level of exposure for either compound (i.e., MCLG zero), community and individual interests in improved drinking-water facility processes and household treatment options are reasonable. The benchmark-weighted screening (Σ TQ ) results (Table S13) also indicate that exposures to inorganics often at concentrations above NPDWR MCLequivalent concentrations are frequent drivers of potential human-health concerns for private-supply TW across the study area, along with more sporadic detections of Pb and other anthropogenic contaminants.
Thus, these results substantiate continued expansion and connection of public-supply systems to limit contaminant exposures to <MCL concentrations and incorporation of wellmaintained POE/POU treatment as an integral complementary line of consumer protection for public-supply TW during normal and outbreak conditions 29,161,164 and as prudent protection against unrecognized simultaneous exposures to multiple contaminants in private-supply homes. 29,41 These results emphasize the importance of continued characterization of POU TW exposures, especially in unregulated and unmonitored private supplies and small community water supplies, using an analytical coverage that serves as a realistic indicator of the breadth and complexity of inorganic and organic contaminant mixtures known to occur in ambient source waters 5,157 to support models of TW contaminant exposures and related risks. Increased availability of such health-based monitoring data, including results below current, technically/ economically constrained enforceable standards (e.g., MCL), is important to support public engagement in source-water protection and drinking-water treatment and to inform consumer POE/POU treatment decisions in these Tribal communities and throughout the US.

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
We thank the Strong Heart Water Study participants and participating Tribal Nations for their support of this drinkingwater research. We thank Jacqueline Bangma (USEPA), Brian Tornabene (USGS), and three anonymous journal referees for their reviews. This research was conducted and funded by the USGS Ecosystems Mission Area, Environmental Health Program. The Strong Heart Water Study team was supported by 1R01ES025135. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The findings and conclusions in this article do not necessarily represent the views or policies of the US Environmental Protection Agency, NIH/National Institute of Environmental Health Sciences, Indian Health Service, or Bureau of Indian Affairs. This report contains CAS Registry Numbers, which is a registered trademark of the American Chemical Society. CAS recommends the verification of the CASRNs through CAS Client ServicesSM.